Elucidation of ABO blood group gene expression though chromatin remodeling

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The ABO blood group is of great importance in blood transfusion and personal identification. However, the mechanisms regulating human ABO gene expression remain obscure. On the basis of DNase I hypersensitive sites in and upstream of ABO in the human erythroleukemia cell line K562 in publicly available expression data from the UCSC Genome Browser (http://genome.ucsc.edu), we prepared reporter plasmid constructs including these sites. Subsequent luciferase assays indicated a novel positive regulatory element in intron 1. This element was shown to enhance ABO promoter activity in an erythroid cell-specific manner. Electrophoretic mobility shift assays demonstrated that it bound to the tissue-restricted transcription factor GATA-1 and GATA-2. Mutation of the GATA motifs to abrogate binding of this factor reduced the regulatory activity of the element. Thus, GATA transcription factors seem to be involved in the cell-specific activity of the element. Furthermore, we found that Bm phenotypes were associated with a partial deletion in intron 1 involving the element as well as a single point mutation of GATA site leading to reduced activity of the element. Therefore, it is plausible that deletion or a single point mutation of the erythroid cell-specific regulatory element could down-regulate transcription in the B^m allele, leading to reduction of B antigen expression in cells of erythroid lineage, but not in mucus-secreting cells. These results support the contention that the enhancer-like element in intron 1 of ABO has a significant function in erythroid cells.